1. Field of the Invention
The present invention relates to a method for separating solids from gas scrubbers.
2. Description of the Related Art
Electrofilters are preferably used as dust collectors in industrial processes in which high off-gas temperatures and large off-gas streams occur, such as in large firing installations, cement furnaces and iron-ore sintering furnaces. Such electrofilters in principle operate in such a way that the released dusts or dust particles are charged in an electric field, the charged dust particles are attracted to a precipitating electrode, where they are then collected.
Power plants that burn fossil fuels to generate energy are subject to legal flue-gas desulfurization requirements. Numerous methods are known for flue-gas desulfurization. Examples worthy of mention here are the limestone-scrubbing method, which is a wet desulfurization method. Usually two-stage scrubber units with prescrubber and main scrubber are used for flue-gas desulfurization.
The first scrubber stage, known as the prescrubber, is operated in fluid having a strongly acid pH, in order to achieve separation of the elements of main group VII of the periodic table. For this purpose, the hot flue gases are cooled in the prescrubber and the acid constituents such as HF and HCl are separated, since fluorine and chlorine in the main scrubber cause serious impairment of the gypsum quality.
A droplet collector is interposed between the prescrubber and the main scrubber in order to prevent entrainment of droplets into the main scrubber and thus to prevent carryover of fluoride and chloride. Depending on fuel composition and electrofilter design, it may be that a specific proportion of the dusts released during combustion is not separated completely in the electrofilter but instead this dust fraction is ultimately washed out in the prescrubber of the flue-gas desulfurization system.
Several reasons can be cited for the increased carryover of ultrafine dust particles into the prescrubber:                change of the usual fuel composition, for example if the sulfur concentration of the fuel composition becomes too low as a result,        overloading of the electrofilter due to increase of the boiler power or to the larger flue-gas stream caused thereby,        general loss of efficiency of the electrofilter, for example due to aging.        
The washed-out dust burden, which accumulates in the form of sludge in the prescrubber, can cause numerous problems.
For example, the carried-over dust particles tend to encrust the plates of the droplet collector usually disposed between the prescrubber and main scrubber and to narrow the flue-gas passage. The direct consequence of this is an increase in the differential pressure Δp across the droplet collector.
Incrustations formed in this way can cause considerable technical and commercial problems, from reduction of the output of the power-plant unit to shutdown of the power-plant unit.
For example, incrustations that have formed from the sludge phase of the prescrubber can lead to fouling of the droplet collector.
Such fouled droplet collectors then force the power-plant operator to reduce the output of the power plant, because the entire flue-gas quantity can no longer be passed through the flue-gas desulfurization system.
The traditional approach to solving this problem is to perform laborious cleaning operations on the droplet collector using a high-pressure water jet. In cases in which the incrustation is too heavy, however, the only and very costly alternative is to replace the droplet collector.
Cleaning of the droplet collector merely treats the symptoms of this problem, however, and does not represent a solution. And this is so, not only because this laborious and cost-intensive cleaning operation has to be performed periodically, but also because such use of high-pressure water jets for each cleaning operation leads to roughening of the material of the droplet collector. In turn, this increased roughness of the droplet collector favors renewed incrustation by the particles and thus increases the necessary cleaning frequency.
Thus, there exists a great need, as regards separating solids from gas scrubbers, for a method that does not suffer from the aforesaid disadvantages, that can be operated at low technical, commercial and labor costs and that, moreover, can be integrated into existing desulfurization systems.